Device for high-frequency heating



June 1956 K. A. BISTERFELD DEVICE FOR HIGH-FREQUENCY HEATING Filed May 16, 1952 2 Sheet s-Sheet 1 Korl Augus'r Bisferfeld E 5 //v VE/V TOR June 5, 1956 K. A. BISTERFELD DEVICE FOR HIGH-FREQUENCY HEATING 2 Sheets-Sheet 2 Filed May 16, 1952 INVENTOR Karl August Bi sterjeld United States Patent DEVICE FOR HIGH-FREQUENCY HEATING Karl August Blister-fold, Hamburg-Volksdorf A, Germany,

assignor to Harfiord National Bank and Trust Company, Hartford, Conn., as trustee Application May 16, 1952, Serial No. 288,286 Claims priority, application Germany August 1, 1951 Claims. (Cl. 21910.43)

This invention relates to a device for high-frequency heating by means of an inductor (working coil) and an associated body for concentrating the lines of force of a ferromagnetic material having a high electric resistance (preferably more than 1009 ohm-cm. and more particularly more than ohm-cm.) for high frequencies, preferably ferrites.

According to the invention, a disc-shaped body, more particularly in the form of a ring, is arranged at least on one side outside the inductor in an axial direction thereof so that the plane of the disc is substantially at right angles to the said axial direction. This ensures on the one hand the advantage that the righ-frequency field is only active in a predetermined, limited zone and on the other that the lines of force are screened in an axial direction of the inductor with the result that definite parts of the workpiece are prevented from being heated unduly. Such a device is particularly useful for sealing together component members of insulating material, such for example as glass or ceramic material, requiring the sealing zone to be limited. The seal may have applied to it as an intermediate layer a substance which in a high-frequency field is heated more readily or has a lower softening point than the material to be sealed, the high concentration of the high-frequency field enabling the intermediate layer to be made from a material which, as regards its electrical and thermal properties, is little different from the material to be sealed. This results in greater liberty as regards the choice of the material of this intermediate layer. In the manufacture of discharge tubes such concentrated heating is also suitable for gettering, unwanted heating of other component members in the tube being thus avoided.

According to a further aspect of the invention, a similar disc-shaped body is also arranged in the same manner at the other end of the inductor. Apart from the said local concentration and screening in an axial direction, this ensures additional concentration in a small zone within the inductor.

Use is preferably made of bodies constituted by rings in each of which a slit (of width, say, 1 mm. at least) is formed so that the rings are open and thus prevented from acting as short-circuit windings.

In one embodiment of the invention in which use is made of two bodies constituted by rings a tubular, preferably slitted, slidable body of high-frequency iron is arranged in the opening of at least one of the rings.

In order that the invention may be clearly under tood and readily carried into effect, it will now be described in detail with reference to the accompanying drawings, in which several embodiments of the invention as shown diagrammatically, by way of example.

Fig. l is a sectional view of a high-frequency coil illustrating that the magnetic lines of force produced by such a coil have an approximately circular route and thus become manifest in a manner not simply controllable beyond the longitudinal dimension of the coil.

According to the invention, as shown in Fig. 2, provision is made at least at one end of such a high-frequency coil 1 of a ring 2 of high-frequency iron, preferably slitted, so that-as shown in the figurethe lines of force 3 are compelled to make their way through the high-frequency iron so that the occurrence of any field below the iron ring is substantially avoided. Obviously, such an iron ring 2. may alternatively be provided at the upper end of the coil or at both ends, if the field is required to be limited upwardly or both upwardly and downwardly.

Devices according to the invention can be utilised with particular advantage in the manufacture of electric discharge vessels. Thus, for example, it has been found, that the gettering operation is efiected in a considerably shorter period of time if use is made of devices according to the invention and that thermal over-straining of other system parts can thus be avoided perfectly. As parts liable to be thermally damaged we may mention, for example, the oxide cathodes of amplifier valves.

The rings 2 should have formed in it a slit at least about 1 mm. in Width, since otherwise they would act as a short-circuit winding and would be heated.

Devices according to the invention may also be used with advantage for other purposes, for example, for firing component members or sealing-in electrical discharge vessels or the like.

Fig. 3 is a diagrammatic sectional view of a device according to the invention which is particularly adapted to seal together articles of annular section, for example, the glass pinch and the glass bulb of an electronic valve.

Fig. 4 shows the manner in which the parts of a tube required to be sealed together are preferably arranged in the high-frequency field.

Fig. 5 shows a ring adapted to be used for preliminary heating.

Fig. 6 shows the construction of a ring of highfrequency iron by which the studs of an electronic valve are protected from unwanted heating The coil 4 shown in Fig. 3 comprises eight copper tube windings wound in two layers which have a cooling agent flowing through them from 5 to 6 via elastic tubes not shown, whilst at the same time a high-frequency current of, say, amp. is passed through. This coil is gripped between two insulating layers 7 and 8 which engage discs 9 and 10 of high-frequency iron which in turn are engaged by insulating plates 11 and 12. The assembly is compressed and mechanically held by bolts 13 and nuts 14. But for the discs 9 and it) the field of the coil 4 would extend approximately as indicated by the two dotted lines 15. An approximately homogeneous field would therefore be produced in the inner space of the coil so that articles introduced into the field would be heated almost evenly. However, according to the invention, provision is made of disc-shaped cores or pole shoes 9 and 10 of high-frequency iron under the action of which the high-frequency field is distorted so that the lines of force extend almost parabolically within the coil (broken lines 16). The field is consequently concentrated and intensified within the coil, whereas the external space of the coil remains substantially free from field. Such an arrangement may be used with advantage in order to vaporise the getter material in a vacuum vessel, for example an electronic valve, since in this case it is frequently desirable that the heating should be limited to the support for the getter material and may be kept with drawn from the other parts of the electrode system.

Fig. 4 shows the position which the glass bulb 17 and the glass pinch 18 with the studs 19 of an electronic valve are required to occupy in the field (shown in broken lines 16) of a H. F. heating device of the kind shown in Fig. 3 during the sealing-in operation. It may be readily seen that the heating takes place primarily at the pinch edge 20. In order to avoid unwanted stresses in the glass it is generally necessary to preheat a comparatively large part of the glass in the proximity of the seal, preferably with decreasing intensity. This purpose is served particularly by a ring 21, as shown in Fig. 5, which is constituted by the short-circuit ring'proper' 22 and tags 23. To eifect the preliminary heating of the glass the ring is slipped over the article to' be sealed. The high-frequency field heats the ring 22 to incandescence. The heat is transmitted with decreasing intensity by the tags 23 to the adjoining material, primarily by radiation, so that the temperature drop is compensated. Such heat conductors may be provided, if required, also on the other side (downwardly directed in Fig. of the ring 22. Their form may be altered to conform, as desired, with the shape of the workpiece and the required temperature distribution. By suitable shaping and arrangement of the high-frequency iron parts, the field can be shaped in any manner desired and the heating can be directed accordingly.

In order that metal parts, such for example as the con tact studs of the pinch of an electronic valve may be protected from undue heating, they may be surrounded below the pinch 18 in the manner shown in Fig. 6 with a preferably slitted protective sheath 24 (protective ring) of H. F. iron or else in a Well known manner with a closed ring of material of good conductivity, such as copper.

Referring to Fig. 7, the turns of the high-frequency coil are designated 25. Provision is made at both ends of the coil of preferably slitted rings 26, which may be made of high-frequency iron, for example of ferrite material. If required, slitted tubings 27 are inserted in the apertures of the rings 26 so as to be adapted to slide. The drawing shows only one of such tubing but a tubing of this kind may be inserted in the aperture of the upper ring 26 so as to be adapted to slide and this enables the active Zone within the inner space of the coil to be displaced in the desired manner.

It is also common practice to provide amplifier valves with a protective cage generally made of so-called elongation material and to surround therewith the system within the tube. In the methods hitherto adapted for high-frequency heating of amplifier valves it was often found that the upper edge of such cages was torn up as a result of the thermal strain by the high-frequency heating. Devices according to the invention even permit of avoiding with certainty said disadvantage.

What I claim is:

1. A high-frequency heating device for heating a workpiece comprising a hollow induction coil having a central axis, an annular disc-shaped ferromagnetic body having an electrical resistance to high-frequency currents in excess of ohm-cm. for concentrating the magnetic flux produced when high-frequency current traverse said coil at certain areas of said workpiece, said ferromagnetic body being disposed adjacent one end of said coil and in a plane substantially at right angles to said central axis, and a short-circuit ring having heat-dissipating tag disposed within said coil adjacent said workpiece.

2. A high-frequency heating device for heating selected areas of a workpiece comprising a hollow induction coil having a central axis and including a coil turn at one end thereof defining a flat given plane extending at right angles to said axis, and an annular disc-shaped ferromagnetic body having a slit extending completely therea through and an electrical resistance to high-frequentcy currents in excess of 10 ohm-cm. for concentrating the magnetic flux produced when high-frequency currents traverse said coil, said ferrogrnagnetic body being mounted in a plane parallel to and axially spaced from said given plane at said one end wholly outside said coil from an axial direction, the interior of said coil being free of material to enable said workpiece to be positioned therewithin.

3. A high-frequency heating device for heating selected of a workpiece comprising a hollow induction coil having a given diameter, a central axis and including a pair of coil turns disposed at opposite ends thereof defining given parallel planes extending at right angles to said axis, and a pair of annular disc-shaped ferromagnetic bodies each having an electrical resistance to high-frequency currents in excess of 10 ohm-cm. for concentrating the magnetic flux produced when high frequency currents traverse said coil at said selected areas, each of said ferromagnetic bodies being mounted in a plane parallel to and axially spaced from one of said given planes at one end of the coil wholly outside said coil from an axial direction, each of said ferrogmagnetic bodies having an outer diameter exceeding said given diameter and an inner diam- :Iter smaller than said given diameter, each of said ferromagnetic bodies having a slit extending completely therethrough in a direction substantially parallel to said axis.

4. A high-frequency heating device as claimed in claim 3 in which a ferromagnetic ring-shaped member having a slit passing therethrough and exhibiting high electrical resistance to high-frequency currents embraces areas of said workpiece to prevent undue heating thereof.

5. A high-frequency heating device as claimed in claim 3 in which a tubular ferromagnetic body having a slit passing therethrough and extending in the direction of said axis slide-ably engages the aperture of one of said annular disc-shaped bodies.

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